The combustion of fossil fuels (i.e. coal, petroleum and natural gas) is the primary source of power production in transportation and stationary power applications. However, combustion is relatively inefficient compared to advanced processes, due to significant heat and frictional losses during the conversion process. Furthermore, the products of combustion, including NOx, SOx, particulate matter, CO, and CO2, are the subject of current and anticipated regulations. There is therefore an interest in energy conversion technologies that utilize fossil fuel resources more efficiently with less of an environmental impact.
For example, one effective means of using fossil fuels as an energy source is to catalytically reform them into synthesis gas (H2+CO), which can then be converted to electrical power in a solid oxide fuel cell (SOFC). Of the available alternative technologies, no other energy production method offers the combination of clean energy and efficiency provided by a SOFC. Because a wide range of fuels can be used, SOFCs have proven to be versatile, and can be used in such applications as auxiliary power units (APUs) for diesel trucks, as well as decentralized stationary power for commercial and military applications
Three main catalytic reforming reactions can be used to convert hydrocarbon fuels into synthesis gas (H2+CO) for fuel cells: steam reforming (SR), catalytic partial oxidation (CPOX), and autothermal reforming (ATR).
SRCnHm + H2O → H2 + COΔH > 0CPOXCnHm + O2 → H2 + COΔH < 0ATRCnHm + H2O + O2 → H2 + COΔH~0
The choice of reforming reaction depends on the application. For fuel cell applications listed where fast light-off, kinetics, quick dynamic response and compactness are of most benefit, CPOX is generally favored. For applications that favor efficiency, SR is generally favored due to its ability to utilize system heat in the reformer. In other applications, ATR or some other degree of oxidative steam reforming (OSR) is desired because of the ability to control the reformate gas composition and minimize heat transfer limitations.
A wide variety of hydrocarbons can be reformed to produce synthesis gas, including natural gas, coal, gasoline and diesel. Of these fuels, diesel is an attractive and practical choice in many cases, because of its high hydrogen density and well developed distribution infrastructure. Again, the choice is application dependent. However, it is also the most difficult fuel to reform because diesel fuel is a mixture of a wide variety of paraffin, naphthene, aromatic and organosulfur compounds, each of which reacts differently in a CPOX reaction sequence. The specific nature of each of these components, i.e. chain length of n-paraffins, substituents attached to hydrocarbon rings, and degree of saturation of aromatic compounds affects the overall fuel conversion. Some of these constituents are also known to deactivate reforming catalysts through carbon formation and sulfur poisoning.
Thus, the challenge in reforming diesel is to develop a catalyst that can maintain high product selectivity to H2 and CO in the presence of aromatics and sulfur species, while being robust enough to operate at reforming conditions, typically 800-1000° C.
Reforming catalysts have typically been nickel or Group-VIII noble metals supported by various high surface area oxide substrates such as aluminas, silicas, and mixed metal oxides. In some cases, pyrochlores are included as suitable supports for the catalytically active metals. See e.g. U.S. Pat. No. 6,238,816, issued to Cable et al, issued May 29, 2001; U.S. Pat. No. 6,409,940, issued to Gaffney et al, issued Jun. 25, 2002. The metal is dispersed onto the support surface in small crystallites to maximize the amount of active metal exposed. However, this design of the catalyst may be predisposed to carbon formation and deactivation by sulfur. The adsorption of sulfur and carbon has been shown to be structure sensitive. Specifically, both carbon and sulfur adsorption have been linked to the metal cluster size, with larger clusters more prone to deactivation. See Barbier et al, “Effect of presulfurization on the formation of coke on supported metal catalysts,” Journal of Catalysis, 102 (1986), among others.
Oxide-based catalysts such as perovskites (ABO3) have been examined as alternatives to noble metal catalysts for at least the autothermal reforming of a JP-8 fuel surrogate and dry (CO2) reforming of methane by substituting various metals into the A and B sites. See Liu and Krumpelt, “Activity and Structure of Perovskites as Diesel-Reforming Catalysts for Solid Oxide Fuel Cell,” Int. J. Appl. Ceram. Technol., Vol 4 (2), (2005), and see Erri et al, “Novel Perovskite-based catalysts for autothermal JP-8 fuel reforming,” Chemical Engineering Science, 61 (2006), among others. Although the perovskite catalysts did exhibit generally favorable activity and coking resistance in the dry reforming case, analyses following catalytic tests showed that the perovskite structure is not maintained and separation of the active metals from the structure is observed. Findings indicate that the structural changes observed in the catalyst occurred primarily during the initial reduction stage. No indication was given regarding long-term stability of that catalyst system.
In another class of oxide-based catalysts, it has been observed that bulk ruthenate pyrochlores (Ln2Ru2O7: Ln is a lanthanide) are highly active for both dry reforming and CPOX of methane. See, Ashcroft et al., “An in situ, energy-dispersive x-ray diffraction study of natural gas conversion by carbon dioxide reforming,” Journal of Physical Chemistry 97 (1993), among others. However, despite high activity, post-run characterization of this particular catalyst revealed that the bulk Pr2Ru2O7 pyrochlore was not stable under CPOX conditions. Catalytic activity of the material was likely derived from the decomposition of the pyrochlore phase under the reducing reaction conditions, which created a Ru-metal enriched surface and a defect fluorite structure in the bulk due to the increased Pr—Ru ratio in the bulk. Decomposition was also observed on the ruthenate catalysts used for the dry reforming of methane. The structural instability of this particular material is not desirable for reforming reactions like CPOX, and likely occurred as a result of selecting a metal (Ru) to occupy entire B-site that is highly reducible. During the break down of the pyrochlore or oxide-based catalyst structure, Ru or active metal migration to the surface leads to the formation of an essentially supported metal catalyst, which should be avoided due to the increased tendency towards deactivation by carbon and sulfur. There is also a tendency toward further metal migration or vaporization, leading to long-term permanent catalytic activity loss.
Similarly, other A2B2O7 pyrochlores have been disclosed for use as catalysts in the reforming of hydrocarbons. These catalysts are limited to A2B2O7 structures and emphasize the catalytic nature of binary mixed metal oxides. Use of additional dopants at the A and B sites in order to enhance the catalytic nature of the crystal structure are not disclosed. Further, in some cases, the catalytic activity of these binary mixed oxides stems largely from B-site migration and the metal-enriched surface which results. See e.g., U.S. Pat. No. 5,500,149, issued to Green et al, issued on Mar. 19, 1996; U.S. Pat. No. 5,149,464, issued to Green et al, issued on Sep. 22, 1992; U.S. Pat. No. 5,015,461, issued to Jacobson et al, issued on May 14, 1991; U.S. Pat. No. 4,959,494, issued to Felthouse, issued on Sep. 25, 1990.
However, many pyrochlores display chemical and thermal stability with high melting points and show the mechanical strength necessary to accommodate metal substitutions necessary for high catalytic activity. The development of a pyrochlore catalyst with spatially distributed active metal components in a structure that resists decomposition at high reforming temperatures would provide a more durable and effective catalyst compared to simple supported metal clusters. Resistance to decomposition would maintain the spatially distributed active metal components as structural components in the pyrochlore, and significantly minimize the migration of active metal components to the surface. This could largely avoid the undesirable defacto formation of a supported metal catalyst from some initially oxide-based catalyst systems at the reforming conditions, and could greatly reduce the tendency towards deactivation by carbon and sulfur. It would also hold potential as a long-life reforming catalyst.
Accordingly, it is an object of this disclosure to provide a method of catalytically reforming a reactant gas mixture using a pyrochlore catalyst material stable under reaction conditions.
It is a further object of this disclosure to catalytically reform a reactant gas mixture using a pyrochlore catalyst material which maintains high product selectivity to H2 and CO in the presence of aromatics and sulfur species.
It is a further object of this disclosure to catalytically reform a reactant gas mixture using a pyrochlore catalyst material that resists both sulfur poisoning and carbon deposition.
It is a further object of this disclosure to catalytically reform a reactant gas mixture using a pyrochlore catalyst material that minimizes catalytically active metal migration to the surface, leading to the formation of a supported metal catalyst.
It is a further object of this disclosure to catalytically reform a reactant gas mixture using a pyrochlore catalyst material where substitution of elements in A sites and/or B sites creates defects in the crystal structure and improves the lattice oxygen mobility.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.